Interferon is one of the most important factors in defense against infections and regulation of immunity, and it has been used for treatment of type B and type C hepatitis and for immunotherapy of cancer. Interferon is actually the sole drug against type C hepatitis. Because interferon is a polypeptide having a molecular weight of about 20,000, it can be applied only by injection and its neutralizing antibody may arise. Since main object of interferon therapy is chronic diseases, there are clinical problems such as restriction of quality of life by long-period going to hospital and decrease of the effect caused by the generation of neutralizing antibody for interferon. Accordingly, orally applicable interferon inducers are desired.
Double-strand nucleic acids originating from virus or other organisms and high molecular polymers such as poly(I):poly(C) and polycarboxylates have been known as interferon inducers. However, their antigenicity, pollution by pathogens, and biological instability are worried, it is therefore difficult to develop high molecular polymers as oral drugs. Fluorenones, pyrimidinones, and anthraquinones have been studied as low-molecular interferon inducers [Mayer, G. D., et al.: Science, 1970, 169, 1214; Nichol, F. R., et al.: Antimicrob. Agents Chemother., 1976, 9, 433; Stringfellow, D. A., et al.: Antimicrob. Agents Chemother., 1991, 15, 111]. However, their development was abandoned because of their insufficient effect or their toxicity [Reiter, M. A., et al.: J. Leukocyte Biol., 1994, 55, 234]. Although imidazoquinolines are known as low-molecular interferon inducers [EP 145,340], they have a low selectivity to interferon and also induce other cytokines, especially TNF-.alpha. and IL-6.